Power amplifiers include class A, class AB, class B, class C, and class D amplifiers depending on differences of operating points. Class A and class AB amplifiers are often used for audios and “class AB push-pull types” are often employed in which the upper half portion (positive half-period) and the lower half portion (negative half-period) of an alternating signal are operated by respective transistors to realize low current consumption. In the class AB push-pull type, an output signal is generated by driving the upper half portion and the lower half portion by an output transistor with a push-pull connection.
FIG. 1 is a diagram showing an exemplary configuration of an operational amplifier using a conventional class A operation. In FIG. 1, reference numeral 11 denotes a differential amplifier circuit comprising: a differential pair made of two transistors M1 and M2; a current mirror circuit made of two transistors M3 and M4 for receiving an output of the differential amplifier circuit 11 with double ends; and a constant current circuit Ic connected to the differential pair. The pair of transistors M1 and M2 of the differential pair has their gates connected to two input terminals IN1 and IN2.
Additionally, sources of the two transistors M1 and M2 are connected each other and one end of the constant current circuit Ic is connected to their common source. The other end of the constant current circuit Ic is grounded. Drains of the two transistors M1 and M2 are connected to a power supply VDD through the respective transistors M3 and M4. The transistors M3 and M4 are connected each other by a current mirror.
Reference characters R1 and R2 denote bias resistances applying a bias voltage VB to the transistors M1 and M2. Additionally, reference character M5 denotes a source-grounded transistor whose gate is supplied with an output signal of the differential amplifier circuit 11, and which functions as a source-grounded amplifier. The source-grounded amplifier M5 has its drain connected to a constant current circuit Io and an output terminal OUT, while a source of the source-grounded amplifier M5 is connected to the power supply VDD. In this manner, the conventional class A amplifier receives the output of the differential amplifier circuit 11 by the source-grounded amplifier M5.
FIG. 2 shows diagrams illustrating waveforms of an alternating signal output from the output terminal OUT. In the class A amplifier configured as described above, a dynamic range for the upper half portion (positive half-period) U of the alternating signal output from the output terminal OUT is determined by the current supply capability of the source-grounded amplifier M5. Since the source-grounded amplifier M5 can be driven sufficiently by the power supply VDD, it has a large current supply capability. Thus, as for the positive half-period U of the alternating signal, it is possible to ensure a sufficiently large amplitude, as shown in FIG. 2(a).
Meanwhile, a dynamic range of the lower half portion (negative half-period) D of the alternating signal is determined by a current value of the constant current circuit Io. Accordingly, if the current value of the constant current circuit Io is sufficiently large, a large amplitude can be obtained even in the negative half-period D, as shown in FIG. 2(a). On the other hand, if the current value of the constant current circuit Io is small, a large amplitude cannot be obtained in the negative half-period D so that the waveform of the alternating signal is distorted, as shown in FIG. 2(b).
Thus, there is a problem that in order to suppress the distortion of the alternating signal the current value of the constant current circuit Io has to be increased so that current consumption increases. In particular, in accordance with a heavier load (smaller load resistance) connected to the output terminal OUT, distortion occurs in the signal unless the value of the constant current circuit Io is increased. Accordingly, if the load connected to the output terminal OUT becomes heavy in the circuit of FIG. 1, it is quite difficult to achieve low current consumption.
A technique is proposed in which two in-phase and anti-phase output signals are received by having an output of a differential amplifier circuit pass through a current mirror circuit (for example, see Patent document 1).
[Patent document 1]: Japanese Patent Laid-open No. 11-308057